Methane and Climate Change

Dave Reay, Pete Smith, Pete Smith, David Reay, Andre Van Amstel, Dave Reay, Pete Smith, David Reay, Andre Van Amstel

• Publisher: Routledge
• Year: 2010
• eBook ISBN: 9781136541520
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Methane is a powerful greenhouse gas and is estimated to be responsible for approximately one-fifth of man-made global warming. Per kilogram, it is 25 times more powerful than carbon dioxide over a 100-year time horizon -- and global warming is likely to enhance methane release from a number of sources. Current natural and man-made sources include many where methane-producing micro-organisms can thrive in anaerobic conditions, particularly ruminant livestock, rice cultivation, landfill, wastewater, wetlands and marine sediments.

This timely and authoritative book provides the only comprehensive and balanced overview of our current knowledge of sources of methane and how these might be controlled to limit future climate change. It describes how methane is derived from the anaerobic metabolism of micro-organisms, whether in wetlands or rice fields, manure, landfill or wastewater, or the digestive systems of cattle and other ruminant animals. It highlights how sources of methane might themselves be affected by climate change. It is shown how numerous point sources of methane have the potential to be more easily addressed than sources of carbon dioxide and therefore contribute significantly to climate change mitigation in the 21st century.

1

Methane Sources and the Global Methane Budget

Dave Reay, Pete Smith and André van Amstel

Introduction

When the late-18th-century Italian physicist Alessandro Volta first identified methane (CH₄) as being the flammable gas in the bubbles that rose from a waterlogged marsh, he could not have guessed how important this gas would prove to be to human society in the centuries that followed. Today CH₄ is used throughout the world as both an industrial and domestic fuel source. Its exploitation has helped drive sustained economic development and it has long provided a lower-carbon energy alternative to coal and oil. As an energy source it remains highly attractive and much sought after; indeed, energy from CH₄ (in the form of natural gas) has played a major role in the UK meeting its Kyoto Protocol commitments to reduce greenhouse gas emissions by allowing a switch from coal firing of power stations to gas firing. However, CH₄ is now increasingly being considered as a leading greenhouse gas in its own right.

At the time that Volta was collecting his bubbles of marsh-gas, CH₄ concentrations in the atmosphere stood at around 750 parts per billion (ppb) and it was almost a century later that John Tyndall demonstrated its powerful infrared absorption properties and its role as a greenhouse gas. Like the other two main anthropogenic greenhouse gases – carbon dioxide (CO₂) and nitrous oxide (N₂O) – the concentration of CH₄ in the atmosphere has increased rapidly since pre-industrial times. Ice core records, and more recently atmospheric samples, show that since the beginning of the industrial era concentrations have more than doubled to their current high of over 1750 ppb (Figure 1.1). This concentration far exceeds the maximum concentration of CH₄ in the atmosphere at any time in the preceding 650,000 years, and is estimated to be resulting in an additional radiative forcing of about 0.5 watts per metre squared compared to the level in 1750.

Figure 1.1 Atmospheric concentrations of carbon dioxide, methane and nitrous oxide over the last 1000 years

Source: Reproduced with permission from the Intergovernmental Panel on Climate Change.

Though these concentrations are much lower than those of CO₂ (currently ~386 parts per million – ppm), CH₄ is more effective at absorbing and reemitting infrared radiation (heat). Indeed, the global warming potential (GWP) of CH₄ on a mass basis is 25 times that of CO₂ over a 100-year time horizon (see Box 1.1).

Box 1.1 Global warming potential

GWP compares the direct climate forcing of different greenhouse gases relative to that of CO₂. The GWP combines the capacity of a gas to absorb infrared radiation, its lifetime in the atmosphere, and the length of time over which its effects on the earth’s climate need to be quantified (the time horizon). In the case of CH₄ it is also adjusted to take account of indirect effects via the enhancement of tropospheric ozone, stratospheric water vapour and production of CO₂ resulting from its destruction in the atmosphere. So, as CH₄ has an effective climate-forcing lifetime in the atmosphere of only 12 years, CH₄ has a GWP of 72 over a 20-year time horizon, but a GWP of 25 over a 100-year time horizon and 7.6 over a 500-year time horizon.

The GWP figures for CH₄ provided in Box 1.1 represent the latest provided by the Intergovernmental Panel on Climate Change (IPCC) in its Fourth Assessment Report (IPCC, 2007). However, these values have varied between the different reports based on improved understanding of atmospheric lifetimes and indirect effects. As such, slightly lower values of GWP for CH₄ are often to be found in the literature, with the GWP of 21 for CH₄ over a 100-year time horizon provided in the Second Assessment Report (IPCC, 1995) being very widely used and forming the basis for most national greenhouse gas budget reporting and trading. Throughout this book we therefore default to the 100-year GWP of 21 provided in the Second Assessment Report, unless otherwise stated.

Box 1.2 Carbon dioxide equivalents

When attempting to assess the relative importance of CH₄ fluxes and mitigation strategies the concept of carbon dioxide equivalents (CO₂-eq) is often employed to convert CH₄ fluxes into units directly comparable with CO₂. This is done simply by multiplying the mass of CH₄ by its GWP to give the mass in CO₂-eq. Usually the 100-year time horizon GWP value (for example 21 or 25) is used, so a reduction of 1 tonne of CH₄ would be a reduction of 21 or 25 tonnes CO₂-eq respectively. However, where a shorter time horizon is considered, the GWP increases substantially, a reduction of 1 tonne of CH₄ using a 20-year GWP time horizon yielding a cut of 72 tonnes of CO₂-eq. A 100-year time horizon has become the commonly used benchmark for national greenhouse gas emissions budgets and trading.

During the 1990s and the first few years of the 21st century the growth rate of CH₄ concentrations in the atmosphere slowed to almost zero, but during 2007 and 2008 concentrations increased once again. Recent studies have attributed this new increase to enhanced emissions of CH₄ in the Arctic as a result of high temperatures in 2007, and to greater rainfall in the tropics in 2008 (Dlugokencky et al, 2009). The former response represents a snapshot of a potentially very large positive climate change feedback, with the higher temperatures projected at high latitudes for the 21st century increasing CH₄ emissions from wetlands, permafrosts and CH₄ hydrates. It is to this and the myriad other natural and anthropogenic determinants of CH₄ flux to the atmosphere that this book is directed.

Though CO₂ emissions and their mitigation still dominate much of climate change research and policy, recent years have seen increasing recognition that reducing CH₄ emissions may often provide a more efficient and cost-effective means to mitigate anthropogenic climate change. In addition, current projections of greenhouse gas concentrations and resultant climate forcing in the 21st century require an improved understanding of how natural CH₄ sources will respond to changes in climate. Our aim, therefore, is to provide a synthesis of the current scientific understanding of the major sources of CH₄ around the planet and, where appropriate, to consider how these emissions may change in response to projected climate change. We then focus on the range of CH₄ emission mitigation strategies currently at our disposal and examine the extent to which these can be employed in future decades as part of national and international efforts to address anthropogenic climate change.

The global methane budget

The global CH₄ budget is composed of a wide range of sources (see Table 1.1 and also Figure 4.5 in Chapter 4) balanced by a much smaller number of sinks, any imbalance in these sources and sinks resulting in a change in the atmospheric concentration. There are three main sinks for CH₄ emitted into the atmosphere, with the destruction of CH₄ by hydroxyl (OH) radicals in the troposphere being the dominant one. This process also contributes to the production of peroxy radicals, and it is this that can subsequently lead to the formation of ozone and so induce a further indirect climate-forcing effect of CH₄ in the atmosphere. In addition this reaction with OH radicals reduces the overall oxidizing capacity of the atmosphere – extending the atmospheric lifetime of other CH₄ molecules – and produces CO₂ and water vapour. Each year an estimated 429–507Tg (teragram; 1Tg = 1 million tonnes) of CH₄ are removed from the atmosphere in this way.

The other sinks are much smaller, with ~40Tg CH₄ removed each year by reaction with OH radicals in the stratosphere, and ~30Tg CH₄ removed by CH₄-oxidizing bacteria (methanotrophs) in soils that use the CH₄ as a source of carbon and energy. A relatively small amount of CH₄ is also removed from the atmosphere through chemical oxidation by chlorine in the air and in the surface waters of our seas. Though several chapters within this book refer to the global CH₄ sinks in terms of their impact on net CH₄ emissions – in particular the soil CH₄ sink – the focus of this book is on the sources of methane, their determinants and their mitigation. Detailed reviews of the key CH₄ sinks and their role globally can be found in Cicerone and Oremland (1988), Crutzen (1991) and Reay et al (2007).

Table 1.1 Global estimates of methane sources and sinks

Natural sources	Methane flux (Tg CH4 yr^–1)^a	Range^b
Wetlands	174	100–231
Termites	22	20–29
Oceans	10	4–15
Hydrates	5	4–5
Geological	9	4–14
Wild animals	15	15
Wild fires	3	2–5
Total (natural)	238	149–319
Anthropogenic sources
Coal mining	36	30–46
Gas, oil, industry	61	52–68
Landfills and waste	54	35–69
Ruminants	84	76–92
Rice agriculture	54	31–83
Biomass burning	47	14–88
Total, anthropogenic	336	238–446
Total, all sources (AR4)^c	574 (582)	387–765
Sinks
Soils	–30	26–34
Tropospheric OH	– 467	428–507
Stratospheric loss	–39	30–45
Total sinks (AR4)	–536 (581)	484–586
Imbalance (AR4)	38 (1)	–199–281

Note: ^a Values represent the mean of those provided in Denman et al (2007, Table 7.6) rounded to the nearest whole number. They draw on eight separate studies, with base years spanning the period 1983–2001. ^b Range is derived from values given in Denman et al (2007, Table 7.6). Values from Chen and Prinn (2006) for anthropogenic sources are not included due to overlaps between source sectors. ^c Values in parentheses denote those provided in the IPCC Fourth Assessment Report (AR4) as the ‘best estimates’ for the period 2000–2004. Source: Values derived from Denman et al (2007)

Of the many significant sources of CH₄ on a global scale, both natural and anthropogenic, the bulk have a common basis – that of microbial methanogenesis. Though CH₄ from biomass burning, vegetation and geological or fossil fuel sources may be largely non-microbial in nature, understanding the processes that underpin microbially mediated CH₄ fluxes is central to quantifying and, potentially, reducing emissions from all other major sources. In Chapter 2 ‘The Microbiology of Methanogenesis’, Fons Stams and Caroline Plugge review our current understanding of microbial methano-genesis and the interactions between different microbial communities that result in the bulk of CH₄ emissions to the global atmosphere.

Natural sources

Major natural sources i...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Contents
• Dedication
• 1 Methane Sources and the Global Methane Budget
• 2 The Microbiology of Methanogenesis
• 3 Wetlands
• 4 Geological Methane
• 5 Termites
• 6 Vegetation
• 7 Biomass Burning
• 8 Rice Cultivation
• 9 Ruminants
• 10 Wastewater and Manure
• 11 Landfills
• 12 Fossil Energy and Ventilation Air Methane
• 13 Options for Methane Control
• 14 Summary
• Contributors
• Acronyms and abbreviations
• Index